This invention relates to thermoplastic shrink films, and particularly thermoplastic shrink films which provide a very high oxygen barrier. More particularly, this invention relates to a thermoplastic coextruded oriented shrink film which provides very high oxygen barrier properties.
Ethylene vinyl alcohol copolymer (EVOH) is well known as an oxygen barrier resin suitable for use in multi-layer films. It is also recognized that vinylidene chloride copolymers, commonly known as SARAN also exhibit oxygen barrier characteristics in a multi-layer film. However, the characteristics of these barrier materials are such that their effectiveness as oxygen barriers is effected by the humidity of the environment in which the film is used, i.e. the barrier properties of these barrier materials are humidity sensitive. The dependence of EVOH on humidity in estimating its gas barrier properties is discussed in the article Ethylene Vinyl Alcohol Resins for Gas-Barrier Material by T. Iwanami and Y. Hirai. This article discusses the degradation in oxygen barrier properties of the EVOH as humidity increases.
Vinylidene chloride copolymers (SARAN) typically exhibit the opposite behavior, with oxygen barrier properties improving somewhat with an increase in humidity conditions.
The combination of EVOH and saran in a coextruded multi-layer film is therefore desirable. This would reduce the overall effect of changes in humidity on the oxygen barrier characteristics of the film.
An additional problem in the use of EVOH, with or without a separate saran layer in a multi-layer structure, is.the cracking problem associated with EVOH. In the past, this cracking problem has been somewhat alleviated by blending EVOH with nylon. The nylon also contributes to ease in orientation of a film containing an EVOH resin. Such processing advantages are made, however, at the expense of higher oxygen transmission rates of EVOH/nylon blends compared with EVOH alone.
Still another problem with unblended EVOH is the difficulty in orienting coextruded EVOH out of a hot water system where EVOH is an inner layer. The EVOH has significantly higher orientation temperature ranges than saran, and when both materials are utilized within a single multi-layer film, an incompatibility in orientation temperature ranges results. While a saran material orients relatively easy after being passed through a hot water system, the EVOH inner layer does not respond as well, and difficulty in orientation results. This is particularly true in the blown bubble technique for orientation well known in the art for producing biaxially oriented film useful in many shrink film applications.
An additional problem with "trapped" EVOH, i.e. EVOH forming an interior layer of a multi-layer structure is the difficulty in removing water from the layer once wetting has occurred. Of course, this difficulty in drying trapped EVOH results in a corresponding loss in oxygen barrier properties because of the poorer barrier performance of EVOH at higher relative humidities.
A past solution to the problem of orienting EVOH out of hot water has been the use of relatively high mole percents of ethylene in the EVOH copolymer. Typically, EVOH resins having between 32 and 38 mole percent ethylene have been employed, and especially those at the higher end of this range, to permit EVOH to approach more closely the orientation characteristics of ethylene vinyl acetate copolymer, and therefore orient more easily.
While the higher ethylene content EVOH resins solve to some extent the problem of orienting an EVOH containing film out of hot water, they also suffer from the disadvantage of reduced barrier properties. As a general rule, the higher the ethylene content in the EVOH, the lower the oxygen barrier properties of the resin, in films employing the resin.
It has now been found that EVOH may be coextruded as an outside layer in a multi-layer film having for example at least one additional polyolefin layer and/or a saran layer, and the coextruded film can be passed through a hot water system and thereafter oriented. The EVOH is wetted and softened by the hot water during the passage of the coextruded film through the hot water. This wetting and softening effect significantly reduces the orientation temperature range of the EVOH to render it much more compatible with the saran layer of the structure if one is present, and in any case to lower the orientation temperature range of the EVOH significantly. Blending of minor amounts of nylon for processing purposes is unnecessary.
Such a reduction in the orientation temperature range of EVOH results in efficiencies in processing compared to a hot oil or hot air orienting system.
Although the EVOH is obviously wetted during the orientation process, its position as an outside layer of the multi-layer film permits relatively rapid drying of the same layer to substantially restore its oxygen barrier characteristics.
An additional advantage is that by employing this method, EVOH resins with a lower mole percent of ethylene can be used, and therefore a higher barrier EVOH resin can be employed.
For extrusion purposes, it is preferable to use a relatively high melt index EVOH resin as an outside layer. Melt indexes of at least 15 grams per 10 minutes (ASTM D-1238) and more preferably at least 20 grams per 10 minutes are preferred. With a melt index of less than about 15 grams per 10 minutes, extrusion becomes increasingly difficult. At melt indexes below about 10 grams per 10 minutes, coextrusion of a multi-layer film with the EVOH as an outside layer becomes very difficult or impossible.
It is an object of the present invention to provide a method of using lower ethylene content EVOH resins in multi-layer shrink films in order to obtain the better oxygen characteristics of these lower ethylene content resins.
Its a further object of the present invention to provide a method for orienting EVOH-containing multi-layer films at relatively low temperatures.
It is yet another object of the present invention to provide an EVOH-containing multi-layer oriented shrink film wherein the EVOH can dry relatively easily compared to trapped EVOH-containing structure.